A 4-year-old boy is brought to the pediatric PA by his parents because they are concerned he is not growing as fast as other children his age. The patient was adopted, and his family history is unknown. Past medical history is unremarkable, and the patient does not have any history of infections. Temperature is 37.6°C (99.7°F), pulse is 86/min, respirations are 27/min, and blood pressure is 115/74 mmHg. On physical examination, the patient’s height is below the 5th percentile and weight exceeds the 75th percentile. Cardiopulmonary examination is unremarkable. Examination of the bilateral hands reveals hypoplastic 4th and 5th digits. The patient’s intelligence quotient (IQ) is well below other children of his age. Laboratory testing reveals the following findings: Laboratory value Result Serum sodium 140 mmol/L (reference range: 136-146 mmol/L) Serum potassium 4.5 mmol/L (reference range: 3.5-5 mmol/L) Serum chloride 97 mmol/L (reference range: 95-105 mmol/L) Serum bicarbonate 25 mmol/L (reference range: 22-28 mmol/L) Serum calcium 9.3 mmol/L (reference range: 8.4-10.2 mmol/L) Serum phosphorus 4.1 mmol/L (reference range: 1-1.5 mmol/L) Which of the following conditions has the same inheritance pattern as this patient’s condition?

With hypoparathyroidism, “hypo” refers to under, and “parathyroid” refers to the parathyroid glands, so hypoparathyroidism refers to a condition where there is an underproduction of parathyroid hormone.

Parathyroid hormone comes from the parathyroid glands which are buried within the thyroid gland, and their main job is to keep blood calcium levels stable.

Now, the majority of the extracellular calcium, the calcium in the blood and interstitium, is split almost equally between two groups - calcium that is diffusible and calcium that is not diffusible.

Diffusible calcium is small enough to diffuse across cell membranes and is separated into two subcategories.

The first is free-ionized calcium, which is involved in all sorts of cellular processes like neuronal action potentials, contraction of skeletal, smooth, and cardiac muscle, hormone secretion, and blood coagulation, all of which are tightly regulated by enzymes and hormones.

The second category is complexed calcium, which is where the positively charged calcium is ionically linked to tiny negatively charged molecules like oxalate, which is a small anion that are normally found in our blood in small amounts.

The complexed calcium forms a molecule that’s electrically neutral but unlike free-ionized calcium it’s not useful for cellular processes.

Both of these are called diffusible because they’re small enough to diffuse across cell membranes.

Finally there’s the non-diffusible calcium which is bound to negatively charged proteins like albumin.

The resulting protein-calcium complex is too large and charged to cross membranes, leaving this calcium also uninvolved in cellular processes.

Changes in the body’s levels of extracellular calcium are detected by a surface receptor in parathyroid cells that’s called the calcium-sensing receptor.

These changes affect the amount of parathyroid hormone that’s released by the parathyroid gland.

The parathyroid hormone gets the bones to release calcium, it gets the kidneys to reabsorb more calcium so it's not lost in the urine, and it synthesizes calcitriol, which is also known as 1,25-dihydroxycholecalciferol, or active Vitamin D.

Active Vitamin D then goes on to cause the gastrointestinal tract to increase calcium absorption.

Altogether, these effects help to keep the extracellular levels of calcium within a narrow range that’s between 8.5 to 10 mg/dl.

The most common cause of hypoparathyroidism is removal of the parathyroid glands during thyroid or parathyroid surgery.

Another cause includes autoimmune polyendocrine syndrome type 1 which destroys parathyroid glands.

Key Takeaways

Hypoparathyroidism is a condition characterized by the underproduction of parathyroid hormone (PTH). PTH is responsible for maintaining the correct levels of calcium and phosphorus in the blood. When PTH levels are low, calcium levels in the blood can drop, which can lead to symptoms like muscle weakness, cramps, and abnormal heart rhythms. Hypoparathyroidism is often caused by the surgical removal of the parathyroid glands, autoimmune destruction, or genetic conditions like DiGeorge syndrome.

Sources

"Robbins Basic Pathology" Elsevier (2017)

"Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)

"Pathophysiology of Disease: An Introduction to Clinical Medicine 8E" McGraw-Hill Education / Medical (2018)

"CURRENT Medical Diagnosis and Treatment 2020" McGraw-Hill Education / Medical (2019)

"Harrison's Endocrinology, 4E" McGraw-Hill Education / Medical (2016)

"Hypoparathyroidism in the adult: Epidemiology, diagnosis, pathophysiology, target-organ involvement, treatment, and challenges for future research" Journal of Bone and Mineral Research (2011)

"Clinical Practice Guidelines for Hypothyroidism in Adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association" Thyroid (2012)

"Treatment for primary hypothyroidism: current approaches and future possibilities" Drug Design, Development and Therapy (2011)

Hypoparathyroidism

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Laboratory value	Result
Serum sodium	140 mmol/L (reference range: 136-146 mmol/L)
Serum potassium	4.5 mmol/L (reference range: 3.5-5 mmol/L)
Serum chloride	97 mmol/L (reference range: 95-105 mmol/L)
Serum bicarbonate	25 mmol/L (reference range: 22-28 mmol/L)
Serum calcium	9.3 mmol/L (reference range: 8.4-10.2 mmol/L)
Serum phosphorus	4.1 mmol/L (reference range: 1-1.5 mmol/L)